This invention relates to a catalytic cracking operation utilizing a C.sub.3-4 paraffin-rich co-feed as a source of light olefins for subsequent conversion to gasoline boiling range components and further features the use of a mixed catalyst system comprising, as a first component, an amorphous cracking catalyst and/or a large pore crystalline cracking catalyst, e.g., zeolite Y, and, as a second component, zeolite Beta and/or a shape selective medium pore crystalline silicate zeolite catalyst, e.g., zeolite ZSM-5.
In known and conventional fluidized catalytic cracking processes, a relatively heavy hydrocarbon feedstock, e.g., a gas oil, admixed with a suitable cracking catalyst, e.g., a large pore crystalline silicate zeolite such as zeolite Y, to provide a fluidized suspension is cracked in an elongated reactor, or riser, at elevated temperature to provide a mixture of lighter hydrocarbon products. The gasiform reaction products and spent catalyst are discharged from the riser into a separator, e.g., a cyclone unit, located within the upper section of an enclosed stripping vessel, or stripper, with the reaction products being conveyed to a product recovery zone and the spent catalyst entering a dense catalyst bed within the lower section of the stripper. In order to remove entrained hydrocarbon product from the spent catalyst prior to conveying the latter to a catalyst regenerator unit, an inert stripping gas, e.g., steam, is passed through the catalyst where it desorbs such hydrocarbons conveying them to the product recovery zone. The fluidized catalyst is continuously circulated between the riser and the regenerator and serves to transfer heat from the latter to the former thereby supplying the thermal needs of the cracking reaction which is endothermic.
Particular examples of such catalytic cracking processes are disclosed in U.S. Pat. Nos. 3,617,497, 3,894,932, 4,309,279 and 4,368,114 (single risers) and U.S. Pat. Nos. 3,748,251, 3,849,291, 3,894,931, 3,894,933, 3,894,934, 3,894,935, 3,926,778, 3,928,172, 3,974,062 and 4,116,814 (multiple risers).
U.S. Pat. No. 3,894,932 describes a single riser fluid catalytic cracking operation in which a gas oil and a C.sub.3-4 -rich gaseous material is converted to aromatics and isobutane in the presence of a faujasite-type zeolite, e.g., zeolite Y.
U.S. Pat. No. 3,894,935 describes a dual riser fluid catalytic cracking process in which a gas oil is catalytically cracked in a first riser in the presence of a faujasite-type zeolite such as zeolite Y to provide gasoline boiling-range material and a C.sub.3-4 -rich hydrocarbon fraction or isobutylene is converted in a second riser in the presence of hot regenerated catalyst or catalyst cascaded thereto from the first riser to provide aromatics, alkyl aromatics and low boiling gaseous material.
Several of the processes referred to above employ a mixed catalyst system with each component of the system possessing different catalytic properties and functions. For example, in the dual riser hydrocarbon conversion process described in U.S. Pat. No. 3,894,934, a heavy hydrocarbon first feed, e.g., a gas oil, is cracked principally as a result of contact with a large pore crystalline silicate zeolite cracking catalyst, e.g., zeolite Y, to provide lighter products. Spent catalyst is separated from the product stream and enters the dense fluid catalyst bed in the lower section of the stripping vessel. A C.sub.3-4 olefin-rich second feed, meanwhile, undergoes conversion to cyclic and/or alkylaromatic hydrocarbons in a second riser, principally as a result of contact with a shape selective medium pore crystalline silicate zeolite, e.g., zeolite ZSM-5. Spent catalyst recovered from the product stream of the second riser similarly enters the dense catalyst bed within the stripper vessel. U.S. Pat. No. 3,894,934 also features the optional introduction of a C.sub.3 -containing hydrocarbon third feed along with an aromatic-rich charge into the dense fluid bed of spent catalyst above the level of introduction of the stripping gas to promote the formation of alkyl aromatics therein. As desired, the third feed may be light gases obtained from a fluid cracking light ends recovery unit, virgin straight run naphtha, catalytically cracked naphtha, thermal naphtha, natural gas constituents, natural gasoline, reformates, a gas oil, or a residual oil of high coke-producing characteristics.
In this and other fluidized catalytic cracking operations employing mixtures of large and medium pore size crystalline silicate zeolite catalysts where catalyst separated from the product effluent is conveyed to a stripper and from there to a catalyst regenerating zone, regardless of the nature of the catalyst introduction at start-up, once steady-state operation has been achieved, the two types of catalyst will become fairly uniformly mixed and will circulate throughout the system at or about the same rate. This arrangement is subject to a significant disadvantage. While the large pore zeolite cracking catalyst cokes up relatively quickly and must therefore be regenerated at frequent intervals, this is not the case with the medium pore zeolite catalysts which can maintain their catalytic activity over many more cycles of operation. However, since the large and medium pore zeolite catalysts are in intimate admixture, heretofore there has been no practical means of conveying only the large pore zeolite catalyst to the catalyst regenerator unit or what amounts to the same thing, keeping the medium pore zeolite catalyst, or at least most of it, on the average out of the regenerator.
Thus, a principal disadvantage resulting from the use of mixed catalyst systems in known fluidized catalytic cracking operations is owing to the fact that the medium pore zeolite catalyst component is subjected to the harsh hydrothermal conditions of the catalyst regenerator unit even though it does not require regeneration anywhere near the rate at which the large pore zeolite cracking component must be regenerated. The medium pore zeolite catalyst is therefore needlessly subjected to hydrothermal deactivation at a much greater rate than is necessary for it to function.
In addition to the foregoing disadvantage, known catalytic cracking processes employing mixed catalyst systems are incapable of carrying out stripping and/or any other operation under conditions which differ for each catalyst component. While, for example, it might be advantageous to strip the spent large pore crystalline silicate zeolite cracking catalyst with steam and the medium pore crystalline silicate zeolite with hydrogen gas so as to also reactivate the latter, heretofore it has not been possible to accomplish this as both catalyst components remain substantially uniformly admixed throughout the entire catalytic cracking system.
U.S. Pat. No. 4,116,814 describes a multiple riser fluidized catalytic cracking operation utilizing a mixture of large and medium pore crystalline zeolite catalysts which differ in particle size and/or density as to facilitate their separation in a common catalyst regeneration unit. There is, however, no hint in this patent of preventing the transfer or reducing the rate of circulation of medium pore crystalline zeolite to and through the catalyst regeneration unit.
U.S. Pat. No. 4,287,088 describes a process and system for the segregation of used contaminated catalyst into fractions according to particle density differences. No mention is made of mixed catalyst systems.